1. Field of the Invention
The present invention relates to a photothermographic material, and in particular, relates to a photothermographic material using a hydrophilic binder as an image forming layer binder.
2. Description of the Related Art
In recent years, in the field of medicine, there has been a strong desire for reductions in the amount of process waste fluids in view of environmental protection and space saving. There is therefore a need for development of technologies concerning photothermographic materials for medical diagnosis and for photographic technology use which can be efficiently exposed using a laser image setter or laser imager and enable the formation of a clear black image having high resolution and sharpness. These photothermographic materials can avoid the use of solvent type processing chemicals and supply to customers a heat developing process system that is simpler and does not impair the environment.
Although there are similar demands in the field of general image forming materials, fine drawing is required for medical images. High image quality superior in sharpness and granularity is therefore necessary in medical images and a cool black tone is preferred in these medical images from the viewpoint of ease of diagnosis easiness. Various types of hard copy systems utilizing pigments and dyes such as ink jet printers and electrophotography are in circulation as general image forming systems. However, none of these satisfy the requirements for medical image output systems.
On the other hand, thermal image forming systems utilizing organic silver salts are described in many documents. Particularly, photothermographic materials generally comprise an image forming layer in which a catalytic amount of a photocatalyst (for example, silver halides), a reducing agent, a reducible silver salt (for example, organic silver salts) and as required, a tone agent that controls the tone of silver, are dispersed in a binder matrix. The photothermographic material is heated to a high temperature (for example, 80° C. or more) after imagewise exposure to form a black silver image by the redox reaction between the silver halide or the reducible silver salt (functioning as an oxidant) and the reducing agent. The redox reaction is promoted by the catalytic action of the latent image of the silver halide formed by exposure. Therefore, the black silver image is formed in the exposed area.
Heat developing treatment has the advantage that the processing solution used in wet developing treatment is unnecessary and the treatment can be carried out simply and rapidly. However, unsolved problems not seen in wet developing treatment exist in heat developing treatment. One of these problems is a problem concerning decoloring of a dye. It is common to add dyes to photosensitive materials for the purpose of making a filter and preventing halation and irradiation. The dye functions in imagewise exposure. If the dye is left in the photosensitive material after this function is finished, the image to be formed is colored with the dye and it is therefore necessary to remove the dye from the photosensitive material in the developing treatment. In wet developing treatment, a dye can be removed from a photosensitive material simply by a processing solution. In heat developing treatment, on the other hand, removal of the dye poses a significant problem.
Particularly in the case of photosensitive materials to be exposed to laser light, it is important to include a sufficient irradiation preventive effect and halation preventive effect at the exposure wavelength in the photosensitive materials in order to obtain a highly sharp image. As the laser wavelength, a wide wavelength range of from the near-infrared region, the infrared region, and the red to blue visible regions are used at present.
A method of discoloring dyes by heating in the heat developing treatment has been proposed. For example, U.S. Pat. No. 5,324,627 discloses a method of discoloring a polymethine dye having a specific structure by heating.
Also, methods by which a polymethine dye is discolored under heating by using a carbanion generating agent have been disclosed (see, for example, U.S. Pat. Nos. 5,135,842, 5,314,795 and 5,324,627).
Problems posed when using a discoloring mechanism are that a dye is insufficiently discolored or, on the contrary, the stability of a dye is insufficient so that the dye is discolored while the photothermographic material is stored. Also, when using a polymethine dye, the decomposed products of the dye left after the dye is discolored have the capability of slightly absorbing light, posing a problem concerning the residual color of an image (particularly in the highlight portion). There is also a problem concerning recoloring (particularly when the dye is brought into contact with an acid) after heat developing and there are cases where byproducts generated after the complicated reaction processes deteriorate the handling characteristics of the light-sensitive material after the photosensitive material is treated.
With regard to photosensitive materials to be exposed by a near-infrared or infrared laser, photosensitive materials have been proposed which substantially exclude a discoloring mechanism by using a dye that has an absorption maximum in the invisible near-infrared region, have a narrow half width and have small absorption in the visible region (see, for example, Japanese Patent Application Laid-Open Nos. 9-146220 and 11-228698).
However, it is commonly known that the tint of a dye is greatly changed by the binder and materials coexisting therewith in a layer having the dye. As the binder of a photothermographic material, a binder, which dissolves in an organic solvent, such as polybutyral or a water dispersion of polymer micro-particles is used. The latter method does not require any process for recovering solvents, which makes production equipment simple and has a reduced environmental load, which is advantageous in mass-production. However, this method has the problem that because the coating solution has no setting ability, the formed film is disturbed by dry air after it is applied, causing drying unevenness. In order to improve this, use of a hydrophilic binder such as a gelatin has been proposed (see, for example, U.S. Pat. Nos. 5,314,795 and 5,324,627).